1. Field of the Invention
The present invention relates to x-ray cassettes and computed radiography and related imaging plates. More specifically, the invention relates to a novel cassette devices for supporting an imaging plate or plates and methods of using such cassette devices.
2. Description of Background Art
Traditional x-ray film has been used as part of the x-ray process for many decades. Almost thirty years ago, however, a fundamental innovation created a first version of a re-usable imaging plate that could be used instead of the traditional x-ray film. The x-rays impregnated the imaging plate with energy which, when subsequently exposed to a particular excitation or laser energy, was then released and captured, recreating the image.
The imaging plates are reflective and also erasable allowing repeated exposures particularly if there are no mechanical stresses on the imaging plate material. The use of imaging plates also eliminates the need for x-ray film and film processing. Presently these imaging plates are stored within a cassette assembly. The imaging plate is then removed from the cassette and transported within the scanning device, typically a computed radiography (“CR”) device by a drive mechanism, typically a set of rollers. This roller type reading process necessitates that the phosphorescent plate be flexible. The feed mechanism used for scanning the plates is typically a set of pinch rollers. Over time the flexing and contact with the rollers causes a deleterious effect on the plate resulting in limited reusability. The phosphor materials in the plates do not degrade due to read/write cycles if mechanical and physical stresses do not act on the plate. Additional wear and tear on the plate is caused by the removal and replacement of the plate in a cassette that protects the plate from ambient energy (light) sources during transportation. In addition to causing degradation of the quality of the plate the pinch rollers can slip causing line artifacts and blurring of the image being generated.
The issue of scanning imaging plates in CR and related scanning devices is surprisingly complex. The use of a rigid cassette box type device for scanning x-rays is generally known as is the automatic removal and reloading of x-ray film from and into the cassette. Similarly, the current technology used for automatically removing, reading and replacing a CR plate relative to the supporting cassette is often similar or identical to that of x-ray film. A critical difference between scanning film and CR plates is that the CR plate is removed, read and replaced over and over again, resulting in stresses to and ultimately, potential failure of the CR plate.
To date, advances in film and plate cassettes have focused on methods of removing the plate from the cassette (for example, the CR plate from the x-ray cassette), reading it, and replacing it inside the cassette. Some of these methods may extend or reduce the life of the CR plate through the respective increase or decrease in handling and stress to the plates. In addition to the need to reduce handling of the plates, there is also a need to reduce any bending or other stresses to the plates and particularly to CR plates. Current CR plates have design limitations and constraints that include coatings and materials that may be able to withstand limited deformation through bending and surface contact of pinch rollers. Generally, CR plate photostimulatable phosphor materials do not materially degrade to read/write cycles unless there is mechanical or contact stresses to the plate.
As mentioned, current cassette designs employ read/write cycles that involve removal of the CR plate from the cassette, either by automatic or manual means. Once the plate is removed, it is driven past a scan head, erased and placed back in the cassette. Often times, these designs have cycle or use limitations for the CR plate due to the manual handling of the plate and related stresses and or damage to the plate due to the scanning process.
Another limitations to the precision of the CR scan process is the mechanism used to drive the CR plate in front of the scan head. If the process depends on pinch rollers, any slippage can cause line artifacts or simply blurring of the image. This error is similar to that found in facsimile machines that rely on pinch rollers to drive the paper past the read head.
The development of new types of CR scanning and reading devices has accelerated in the past few years. For example, a “needle phosphor storage imaging plate” (“Needle IP”) reader has been proposed that can be scanned faster than traditional CR (e.g.: 14 seconds instead of 40 to 60 seconds), with apparently less x-ray dose to the patient, at higher resolution (e.g.: 50 micron resolution from 100 micron). This technology, however, has had many problems with the reader, not the least issue is that the plates are made of delicate crystals of a glass like phosphor material. This phosphor is organized in tall “needles” that provide a high level of signal output when stimulated by laser light. These needles are prone to breaking if the cassette is dropped. What is needed for these advanced imaging plates, along with traditional imaging plates, is a device and method of scanning and processing the retained images without bending and handling stress.